Melanocortin peptides, particularly α-melanocyte stimulating hormone ((α-MSH), have a wide range of effects on biological functions including feeding behavior, pigmentation, and exocrine function. See Wikberg et al., “New Aspects on the Melanocortins and their Receptors,” Pharmacological Research, Vol. 42, No. 5 (2000), at pp. 393–420. The biological effects of α-MSH are mediated by a sub-family of G protein-coupled receptors, termed melanocortin receptors. See Wikberg et al., supra. There are four melanocortin receptors: MC-1R, MC-3R, MC-4R, and MC-5R (MC-2R is not a receptor for α-MSH but is the adrenocorticotropic hormone {ACTH} receptor). Activating any one of these receptors results in stimulation of cAMP formation.
MC-1R was first found in melanocytes. Naturally occurring inactive variants of MC-1R in animals were shown to lead to alterations in pigmentation and a subsequent lighter coat color. From these and other studies, it is evident that MC-1R is an important regulator of melanin production and coat color in animals (or skin color in humans). MC-3R is expressed in the brain and peripheral tissues, and knock-out studies have revealed that MC-3R is responsible for alterations in feeding behavior and body weight. MC-4R is primarily expressed in the brain. Genetic knock-outs and pharmacologic manipulation of MC-4R in animals have shown that agonizing MC-4R causes weight loss and antagonizing MC-4R produces weight gain. MC-5R is ubiquitously expressed in many peripheral tissues and in the brain, but its expression is greatest in exocrine glands. Genetic knock-out of this receptor in mice results in altered regulation of exocrine gland function, leading to changes in water repulsion and thermoregulation.
Much attention has been focused on the study of MC-3R and MC-4R modulators and their use in treating body weight disorders, such as obesity and anorexia. For example, WO 00/74679 to Merck & Co., Inc.,“Substituted Piperidines as Melanocortin-4-Receptor Agonists,” (Dec. 14, 2000), and WO 99/64002 also to Merck & Co Inc., “Spiropiperidine Derivatives as Melanocortin Receptor Agonists,” (Dec. 16, 1999), disclose compounds that reportedly are selective agonists of MC-4R. Each of the compounds of WO 00/74679 and WO 99/64002 has a bicyclic terminal group, typically tetrahydroisoquinoline. Isoquinoline compounds, more particularly tetrahydro-isoquinoline-based compounds, useful as melanocortin (MC) receptor agonists and antagonists are disclosed in U.S. Pat. No. 6,127,381, “Isoquinoline Compound Melanocortin Receptor Ligands and Method of Using Same,” issued Oct. 3, 2000 to Basu et al. See also WO 01/91752, WO 01/70708, and WO 01/70337 to Merck and WO 02/00654 to Pfizer Products Inc., reportedly disclosing compounds for use as MC-4R agonists. Individual compounds can bind to multiple MC receptors, with different levels of affinity. Yet it is advantageous in treating diseases for compounds to be selective for one or more particular MC receptors. See, e.g., WO 00/58361 to Proctor & Gamble Co., “Melanocortin Receptor Ligands” (disclosing compounds that reportedly are selective for MC3R and MC4R in preference to MC-1R); and WO 99/54358 to Quadrant Holdings Cambridge Ltd., “Melanocortin Receptor Ligands” (disclosing compounds that reportedly are selective for MC3R, MC4R and/or MC5R).
The melanocortin peptides have potent physiological effects besides their role in regulating pigmentation, feeding behavior, and exocrine function. In particular, α-MSH has been shown to induce a potent anti-inflammatory effect in both acute and chronic models of inflammatory diseases including inflammatory bowel disease, renal ischemia/reperfusion injury, and endotoxin-induced hepatitis. See Catania et al., “a-MSH in Normal Human Physiology and Disease States,” Trends in Endocrinology and Metabolism, Vol. 11, No. 8 (2000) at pp. 304–308. Administration of α-MSH (either i.p. or i.v.) in these models results in substantial lessening of inflammation-mediated tissue damage, a significant decrease in leukocyte infiltration, and a dramatic reduction in elevated levels of cytokines (e.g., TNF-α), chemokines (e.g., MCP-1, IL-8), and inflammatory mediators (e.g., i-NOS and ICAM-1), to near baseline levels. Earlier studies had shown that α-MSH acts as an “anti-cytokine” in many acute inflammatory models, in effect antagonizing the pro-inflammatory actions of TNF-α, IL-1β, and IL-6.
The anti-inflammatory actions of α-MSH are mediated by MC-1R. MC-1R is expressed in cells that are important regulators of the immune response: monocyte/macrophages, neutrophils, endothelial, and mast cells. See Catania et al., cited above. Stimulation with α-MSH results in a dampening of the inflammatory response in these cells, including inhibition of nitric oxide formation, decreased expression of co-stimulatory molecules and adhesion receptors, and importantly, an increase in the expression of IL-10; a cytokine with potent anti-inflammatory actions. Studies have shown that MC-1R selective peptides are as efficacious as α-MSH in eliciting an anti-inflammatory response. See Wikberg, “Melanocortin Receptors: Perspectives for Novel Drugs,” European Journal of Pharmacology, Vol. 375 (1999), at pp. 295–310, and WO 99/57148 to WA Pharma AB (1999), “Melanocortin 1 Receptor Selective Compounds.”
The mechanism by which agonism of MC-1R results in an anti-inflammatory response is likely through inhibition of the pro-inflammatory transcription activator, NF-κB. NF-κB is a pivotal component of the pro-inflammatory cascade, and its activation is a central event in initiating many inflammatory diseases. In a typical inflammatory response, NF-κB is activated in response to an inflammatory stimulus and once activated, induces expression of a wide array of pro-inflammatory genes. See Tak and Firestein, “NF-κB: a Key Role in Inflammatory Diseases,” The Journal of Clinical Investigation, Vol. 107 (2001), pp. 7–11. Activation of MC-1R, and subsequent generation of cAMP and/or decreased production of nitric oxide, has been shown to inhibit activation of NF-κB. See Manna and Aggarwal, “α-MSH Inhibits the Nuclear Transcription factor NF-κB Activation Induced by Various Inflammatory Agents,” The Journal of Immunology, Vol. 161 (1998), pp. 2873–2880. Thus, α-MSH exerts anti-inflammatory actions through stimulation of MC-1R on cells involved in the inflammatory response and subsequent inhibition of the activation of the pro-inflammatory transcription factor NF-κB. Additionally, studies show that anti-inflammatory actions of α-MSH may be, in part, mediated by agonism of MC-3R and/or MC-5R. See WO 00/05263 to William Harvey Research Limited (2000), “Compounds for Use in the Treatment of Inflammation.”
The present invention provides compounds useful as modulators of the melanocortin receptors, including selective modulators of MC-1R and/or MC-4R. Compounds that reportedly are selective agonists of MC-1R are disclosed in WO 99/57148, cited above, and selective antagonists of MC-1R are disclosed in WO 99/43709 to The Regents of the Univ. of Calif., “Melanocortin Receptor Antagonists and Modulations of Melanocortin Receptor Activity.” Both WO 99/57148 and WO 99/43709 disclose large polypeptides. Small molecule inhibitors are advantageous in comparison to large polypeptides as they are less likely to induce immune reactions in patients and are more amendable to oral delivery. There remains a need for a small molecule useful as an MC-1R agonist, which is provided by the present invention. Melanocortin receptor, modulators are also disclosed in U.S. patent application Ser. No. 10/090582, filed concomitantly herewith by the same inventors herein and assigned to the present assignee, claiming priority to U.S. patent applications Ser. Nos. 60/273,206, and 60/273,291, filed Mar. 2, 2001, the entire contents of which is incorporated herein by reference. Also incorporated herein is WO 99/58501 to Novo Nordisk.